1. Field of the Invention
This invention relates to electronic circuits, and more specifically to short circuit protection circuits, and still more specifically to short circuit protection circuits used to protect a low-side driver transistor.
2. Description of the Relevant Art
The problem addressed by this invention is encountered in short circuit protection circuits which are used to protect power transistors such as the power transistors used to drive inductive loads. FIG. 1, for example, shows power transistors 9 and 10 driving the inductive load 14. Inductive load 14 is the schematic representation of the stator coils, arranged in a wye configuration, of a DC motor. Power transistor 9 is the high-side driver for coil 14A. Similarly, power transistor 10 is the low-side driver for coil 14B. It is understood be persons skilled in the art that the circuitry necessary to run the DC motor is not shown in FIG. 1.
Under normal conditions, the current through the low-side driver is limited because of the impedance of the stator coils. However, stator coils can fail in a manner which short circuits node 13 to a high voltage. The problem addressed by this invention occurs in trying to protect the low side driver 10 from excessive current when pin 13 has been short-circuited to a high voltage.
FIG. 1 illustrates the prior art short circuit protection circuit for the low-side driver 10. In this circuit, a short circuit on node 13 will cause an excessive current through the drain-source channel of low-side driver 10 and through sense resistor 12. The excessive current will increase the voltage drop across sense resistor 12. When the voltage drop across sense resistor 12 exceeds the reference voltage Vref, the output of comparator 4 will change from a low voltage to a high voltage thereby turning on transistor 6. Consequently, current source 8 draws current from the gate of low side driver 10 since current source 8 is larger than current source 2. Thus, transistor 10 is turned off at a controlled slew defined by the difference between current source 2 and current source 8. The slew rate is designed to optimize the response time and yet maintain stability, as is known in the art. However, current source 8, by necessity to add stability to the circuit, slows down the response time of the short circuit protection circuit.
Therefore, it is an object of the invention to provide a stable short circuit protection circuit which has improved response time. It is another object of the invention to improve the response time of the short circuit protection circuit with out changing the stability of the circuit. This and other objects, features, and advantages of the invention will be apparent to those skilled in the art from the following detailed description of the invention, when read with the drawings and appended claims.